Edward Jenner worked as a local practitioner in a time where smallpox was considered the most deadly disease. In 1788 a wave of smallpox swept through Gloucestershire, which is when Jenner observed those who had came in that had been in contact with cowpox, a much milder disease, hadn't gotten smallpox. He just needed to test his theory.

Medical Contribution

Almost ten years later, Jenner conducted an experiment on an eight year old patient, James Phipps. He made two cuts in his arm and worked in amount of cowpox puss. He had a normal reaction with just a slight fever, it within days he was in good health. Weeks later he conducted the same procedure, but with smallpox. The boy remained in good health which is how Jenner's vaccination treatment was born

Riedel (1925), Edward Jenner Biography. (n.d.)